The invention relates to a method for monitoring the function and/or regeneration of a gas probe.
Double chamber NOx limit current sensors for measuring the oxygen or nitrogen oxide concentration in the exhaust gas of motor vehicles have been known for some time and are described, for example, in the publication xe2x80x9cSAE Technical Paper Series No. 960334xe2x80x9d entitled xe2x80x9cThick Film ZrO2 NOx Sensorxe2x80x9d. These sensors have an oxygen ion conducting ceramic, preferably zirconium oxide, which ceramic includes two compartments connected to the exhaust gas of the engine. Several electrodes are mounted in the exhaust gas as well as in the chambers. In a first chamber, the oxygen is electrochemically pumped away by means of two electrodes. This first chamber communicates directly with the exhaust gas of the engine via a porous region of the ceramic. A reduction of the oxygen molecules takes place on the cathode mounted in one chamber. In the oxygen-ion conducting ceramic, the oxygen ions, which arise in this manner, are transported because of the applied voltage to the anode which is mounted in the exhaust-gas flow and where a reversal of the reduction takes place, that is, oxygen molecules are formed from the oxygen ions by adding electrons. The consumed or regenerated electrons can then be measured as an electrical current.
A reduction of the flow density takes place because of deterioration after a longer duration of operation.
It is the task of the invention to provide a method for monitoring the function and/or regeneration of a gas probe of the kind described initially herein which makes possible a regeneration of the gas probe in dependence upon the detected operability.
The task is solved with the features of claim 1. The method of the invention affords the advantage that not only the function capability of the exhaust-gas probe is monitored but that a regeneration of the probe takes place when a deterioration of the exhaust-gas probe is determined. This regeneration is undertaken by applying a regeneration voltage which is greater than the saturation voltage.
The steps for checking the functioning capability of the probe and the application of the regeneration voltage is repeated several times, preferably five times, until there is a drop below a pregiven threshold value. A disturbance announcement is outputted when there is no drop below this threshold value.
As to the regeneration voltage, it can be of the same polarity as the pump voltage but it is also possible that it has a polarity opposite to the pump voltage. The regeneration voltage can be applied in the lean phase, in the rich phase, or even for an air number xcex=1 of the engine.
Advantageously, the flow, which adjusts because of the application of the regeneration voltage, or another index for this current is measured, such as the time during which the regenerating voltage was applied, in order to set a diagnostic signal or to input a value into a fault memory of the sensor part.